Communications network with analysis of detected line faults for determining nodes as probable fault sources

ABSTRACT

A fault management system for an access network forms part of a communications network. In the access network, terminating lines in the form of pairs of wires extend from a local switch through a series of nodes to terminal equipment provided for user of the network. Each night, the system performs a series of tests on each of the lines. The results of the tests then analyzed with respect to a set of parameters to identify characteristics that would indicate that a fault is likely to occur on the associated circuit within a predetermined period e.g. 1 year. Further analysis is then carried out to establish a score which ranks each node in the network in terms of the urgency of any preventive maintenance required.

This application is the US national phase of international applicationPCT/GB02/01123 filed 12 Mar. 2002 which designated the U.S.

RELATED APPLICATION AND PATENTS

This application may be related to the following copending commonlyassigned application and/or patents:

a) U.S. Ser. No. 10/471,080 filed Sep. 8, 2003 naming Richard Maxwell assole inventor entitled “FAULT MANAGEMENT SYSTEM FOR A COMMUNICATIONSNETWORK”

b) U.S. Pat. No. 6,125,458 B1 dated Sep. 26, 2000 naming Ian R. Devan,Andrew D. Chaskell as inventors and entitled “FAULT MANAGEMENT SYSTEMFOR A TELECOMMUNICATIONS NETWORK”

c) U.S. Pat. No. 6,128,753 dated Oct. 3, 2000 naming Peter J. Keeble,Andrew D. Chaskell & Robert D. Bailey as inventors and entitled “FAULTMANAGEMENT SYSTEM FOR A TELECOMMUNICATIONS NETWORK”.

BACKGROUND

1. Technical Field

This invention relates to a fault management system for managing faultsin the terminating circuits of a communications network and also to amethod of operating such a fault management system.

2. Related Art

A conventional communications network comprises a relatively smallnumber of interconnected main switches and a much larger number of localswitches, each of which is connected to one or two of the main switches.The local switches are connected to the terminating circuits of thenetwork and the far ends of these circuits are connected to terminalequipment such as telephone instruments provided for users of thenetwork. The network formed from the main switches and local switches isknown as the core network while a network formed from the terminatingcircuits is known variously as an access network or a local loop. Inthis specification, it will be referred to as an access network. Someterminating circuits are connected to a remote concentrator, which mayor may not have switching capability. The remote concentrator is thenconnected to a local switch. In this specification, the term “localswitch” is to be interpreted to cover both local switches and remoteconcentrators.

In a conventional access network, each terminating circuit is formedfrom a pair of copper wires. Typically, each pair of copper wires passesthrough a series of nodes (or network elements) between the local switchand terminal equipment. Examples of such nodes are primary cross-connectpoints, secondary cross-connect points, distribution points (DPs), cablenodes and joints.

Recently, optical fibres have been used to carry terminating circuits inaccess networks. In a modern access network, both pairs of copper wiresand optical fibres are used to carry the terminating circuits. Where aterminating circuit is carried by an optical fibre, the circuit willtypically pass through several node between the local switch and theterminal equipment. At each node, the incoming fibre from the localswitch is split into a group of outgoing fibres which branch out invarious directions. Where a terminating circuit is carried by an opticalfibre from the local switch, the last part of the circuit may be carriedby a pair of copper wire. Unfortunately, terminating circuits are proneto faults. In the case of a terminating circuit carried by a pair ofcopper wires, example of such faults are disconnection, a short circuitbetween two wires of a pair of wires and a short circuit between one ofthe wires and earth. In the case of a conventional access network formedfrom pairs of wires, the causes of the faults include ingress of waterinto a node and also physical damage to a node.

When a customer reports a fault, the terminating circuit may be testedso as to identify the cause of the fault. The fault can then berepaired. However, until the fault is repaired, the user suffers a lossof service. It is known how to perform a set of circuit tests on eachterminating circuit in an access network on a routine basis, for examplenightly. Such routine tests can detect a fault on a terminating circuit.The fault can then be repaired, possibly before the user of theterminating circuit notices a loss of service. It is also known tomeasure the operational quality of individual nodes of an accessnetwork. Where the operational quality of a node is poor, it is likelythat faults will develop in terminating circuits passing through thenode. However, lines run though a number of nodes before terminating andso as a result, locating the node from which potential faults emanate isdifficult and so efficient preventive maintenance is difficult.

BRIEF SUMMARY

According to an embodiment of the present invention there is provided amethod of operating a fault management system for a communicationsnetwork comprising a plurality of lines passing through a plurality ofnodes, said method comprising the steps of:

-   -   performing a test on a plurality of said lines to obtain one or        more elements of test data for each line;    -   analysing the test data to identify lines with common fault        characteristics; establishing a score for each node based on the        number of lines with common fault characteristics that provides        a relative measure of the urgency of the maintenance tasks        required to remove the fault characteristics from the node.

The priority score gives a relative measure for each node of the urgencyand magnitude of the maintenance of a network node. This enablespreventative maintenance to be targeted at the node most in need of itto avoid the worst consequences.

BRIEF DESCRIPTION OF THE DRAWINGS

An exemplary embodiment of this invention will now be described in moredetail, by way of example, with reference to the accompanying drawingsin which:

FIG. 1 is a block diagram of an access network and an associated localswitch which form part of a communications network in which an exemplaryembodiment of the present invention may be used;

FIG. 2 is a block diagram showing the components of the communicationsnetwork which are used to provide a fault management system in anexample embodying the invention for the access network of FIG. 1;

FIG. 3 is a circuit diagram illustrating some of the measurements whichare made when testing a terminating circuit;

FIG. 4 is a flow diagram illustrating the processing performed in thefault management system in identifying faults in the network;

FIG. 5 is a table of example test data used in an example of the processillustrated in FIG. 4;

FIGS. 6 and 7 are schematic illustrations of a communications showing aplurality of network nodes interconnected by communications lines.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Referring now to FIG. 1, there is shown a local switch 10 and aconventional access network 12 connected to the local switch 10. Thelocal switch 10 and the access network 12 form part of a communicationsnetwork. The local switch 10 is connected to the terminating circuits orlines of the access network 12. Typically, a local switch is connectedto several thousand terminating circuits. Each terminating circuit orline passes through several nodes before reaching its respectiveterminal equipment. These nodes comprise primary cross-connect points,secondary cross-connect points, distribution points (DPs) and junctionsand examples of these nodes will be described below.

In the conventional access network 12 shown in FIG. 1, each terminatingcircuit or line is formed from a pair of copper wires. The copper wiresleave the local switch 10 in the form of one or more cables. One ofthese cables is shown in FIG. 1 and indicated by reference numeral 14.The far end of cable 14 from switch 10 is connected to a primarycross-connect point 16 which may be housed in a street cabinet orunderground junction box. From the primary cross-connect point 16, theterminating lines branch out as cables in several directions. Forsimplicity, in FIG. 1 there are shown only three cables 18, 20 and 22.The far end of cable 18 is connected to a joint 19. The joint 19 isconnected by cable 21 to a secondary cross-connect point 24. The farends of cables 20 and 22 are connected, respectively, to secondarycross-connect points 26 and 28. For reasons of simplicity, thecontinuations of the terminating lines beyond secondary cross-connectpoints 24 and 26 are not shown. The secondary cross-connect points 24,26 and 28 are housed in junction boxes which may be located above orbelow ground.

From the secondary cross-connect point 28, the terminating lines branchout again in several directions in the form of cables. By way ofillustration, FIG. 1 shows cables 40, 42, and 44 leaving the secondarycross-connect point 28. Cables 40 and 44 are connected, respectively, tojoints 46 and 48. Joints 46 and 48 are connected, respectively, tocables 50 and 52, the far ends of which are connected to distributionpoints 54 and 56. The far end of cable 42 is connected to a joint 60.The joint 60 is connected by cable 62 to a distribution point 64. Forreasons of simplicity, the terminating lines beyond distribution points54 and 56 are not shown.

Distribution points are implemented as junctions boxes which aretypically located on telephone poles. From each distribution point, theterminating lines branch out as single copper wire pairs to whereterminal equipment provided for a user of the network is located. By wayof illustration, FIG. 1 shows two single copper wire pairs 70, 72,leaving the distribution point 64. The far ends of copper wire pairs 70and 72 are connected, respectively, to terminal equipment 74, 76. As iswell known, terminal equipment may take various forms. For example,terminal equipment may be a telephone located in a telephone box, atelephone instrument located in a domestic house or an office, or a faxmachine or a computer located in a customer's premises. In the exampleshown in FIG. 1, each of the joints 19, 46, 48 and 60 is used to connecttwo cables together. Joints may also be used to connect two or moresmaller cables to a larger cable.

In each terminating line, the two wires of each pair are designated asthe A wire and the B wire. At the local switch 10, in order to supplycurrent to the line, a bias voltage of 50V is applied between the A wireand the B wire. As the bias voltage was applied in the early exchangesby using a battery, the bias voltage is still known as the batteryvoltage. In the terminal equipment, the A wire and B wire are connectedby a capacitor, the presence of which may be detected when the terminalequipment is not in use.

The terminating lines in the access network 10 are prone to faults. Themain causes of these faults are ingress of water and physical damage tothe nodes through which the terminating lines pass between the localswitch 10 and terminal equipment. There are five main faults which occurdue to causes arising in the nodes. These faults are disconnection,short circuit, faulty battery voltage, earthing fault and low insulationresistance. A disconnection arises where a terminating line isinterrupted between the local switch and the terminal equipment. A shortcircuit arises where the A wire and B wire of a line are connectedtogether. A faulty battery voltage arises where the A wire or the B wireof a terminating line has a short circuit connection to the B wire ofanother line. An earthing fault arises when the A wire or B wire isconnected to earth or the A wire of another line. Low insulationresistance arises where the resistance between the A wire and the B wireor between one of the wires and earth or between one of the wires and awire of another line is below an acceptable value.

In order to detect faults in the terminating lines of the access network12, the local switch 10 is provided with a line tester 80. The linetester 80 may be operated from the local switch 10 or, as will beexplained in more detail below, from a remote location. The line tester80 is capable of performing various tests, examples of which will bedescribed below. Various models of line testers for local switches areavailable commercially. In the present example, the line tester 80 iseither Teradyne and Vanderhoff test equipment. In some case both typesof test equipments may be used. As well as producing resistance,capacitance and voltage measurement data for line these pieces ofequipment also further data called termination statements such as “BellLoop”, “Master Jack Loop” and “Bridged”. These termination statementsare special line conditions which the equipment is arranged to detect.

Referring now to FIG. 2, there is shown the local switch 10 and thecomponents of the communications network which provide a faultmanagement system for the access, network 12. These components comprisethe line tester 80, a customer service system 100 for the communicationsnetwork and an access network management system 102. The line tester 80comprises a test head 104 which contains the electronic equipment forphysically making line tests and a controller 106 for the test head 104.The controller 106 takes the form of a computer. The controller 106 canbe operated from a workstation 108 connected to it and provided at thelocal exchange 10. The controller 106 is also connected to both thecustomer service system 100 and the access network management system 102and can be operated by workstations connected to either the customerservice system 100 or the access network management system 102.

The customer service system 100 is also a computer and it can beoperated from any one of a number of workstations which are connected toit. In FIG. 2, one such workstation is shown and indicated by referencenumeral 110. The customer service system 100 is used by operators of thecommunications network who have contact with the customers of thenetwork. Together with these operators, the customer service system isresponsible for providing various services to the customers.

The access network management system 102 is also a computer and it canbe operated from one of a number of workstations. One of theseworkstations is shown in FIG. 2 and indicated by reference numeral 112.The access network management system 102 is responsible for managing theaccess network 12 as well as a number of other access networks in thesame general geographical area as the access network 12. The accessnetwork management system 102 manages various operations for each of theaccess networks which it manages. These operations include the provisionof new equipment, logging data on work performed by engineers in thenetwork, maintaining data on the terminating lines and nodes of eachaccess network detection and management of faults. The workstationswhich are connected to the access network management system 102 are alsoconnected to the customer service system 100. As shown in FIG. 2, thecustomer service system 100 and the access network management system 102are connected together.

Although in the present example the fault management system for theaccess network 12 is formed from the line tester 80, the customerservice system 100 and the access network management system 102, thefault management system could also be provided simply by the line tester80 on its own. In order to achieve this, it would be necessary to addappropriate software to the computer which forms the controller 106. Ina small network, this might be an appropriate form of providing thefault management system. However, in a large network it is advantageousto integrate the fault management system into the customer servicesystem 100 and the access network management system 102.

The controller 106 is programmed to cause the test head 104 to make aseries of routine tests each night on each terminating line of theaccess network 12. These tests will be explained with reference to thecircuit diagram shown in FIG. 3.

In order to test a line, may be disconnected from the switch 10 andconnected to the test head 104. FIG. 3 shows a line 300 being tested.The line 300 has an A wire 302 and a B wire 304. The end of line 300remote from switch 10 is connected to terminal equipment 306. Each ofthe lines 302, 304 has a resistance which depends upon its diameter andthe distance from the local switch to the terminal equipment 306. Eachof the wires 302, 304 is coated with an insulating material. Thefunction of the insulating material is to provide insulation betweeneach wire and adjacent wires. Damage to the insulating material oroxidation of the metal of a wire can cause the resistance between twoadjacent wires to fall.

The effectiveness of the insulation between wires 302, 304 can bedetermined by measuring the resistance R1 between the A wire 302 and theB wire 304 and the resistance R2 between the B wire 304 and the A wire302. The resistances R1 and R2 may be different because of rectificationas indicated by diodes D1 and D2. For a circuit in good condition, theresistances R1 and R2 are high, greater than 1 megaohm. Damage to theinsulating material or oxidation will cause the resistances R1, R2 tofall by an amount which depends upon the severity of the damage oroxidation. If the insulating material is totally destroyed so that the Aand B wires are physically touching each other, the values ofresistances R1, R2 will depend upon the distance between the test head80 and the point of damage but will typically lie in the range 0 to 1500ohms. Oxidation can result in wires effectively touching each other.

Only the A and B wires 302, 304 of the line 300 being tested aredisconnected. In the other lines, the bias voltage of 50 volts isapplied between the A wire and the B wire. In FIG. 3, the A wires of theother lines are collectively shown by a wire 310 which is connected atthe switch 10 to earth. The B wires of the other lines are collectivelyshown by a wire 312 connected at the switch to a potential of −50 volts.

If the insulating material separating the A wire 302 or the B wire 304from one of the adjacent A or B wires becomes damaged, or if one of thewires suffers oxidation, current may flow. The effectiveness of theinsulation between the A and B wires 302, 304 and adjacent A and B wirescan be determined by measuring the resistance R3 between A wire 302 andadjacent A wires 310, the resistance R4 between the A wire 302 andadjacent B wires 312, the resistance R5 between the B wire 304 andadjacent A wires 310, and the resistance R6 between the B wires 304 andadjacent B wires 312.

For a good circuit, the resistance R3, R4, R5, R6 are high, greater than1 megohm. Damage to insulating material may cause one or more of theresistances R3, R4, R5, R6 to fall by an amount which depends upon theseverity of the damage. If the insulating material between the A wire302 or the B wire 304 and an adjacent wire is totally destroyed so thatthe two wires are physically touching each other, the resistance betweenthe two touching wires will depend upon the distance between the testhead 80 and the point of damage but will typically lie in the range 0 to1500 ohms. Oxidation can also result in two wires effectively touchingeach other.

The A and B wires 302, 304 and the insulating material between them actas a capacitor. In FIG. 3, the capacitance between the A and B wires isshown as having a value C1. The value of the capacitance between the Aand B wires of a line will depend upon the length of the line. A breakin the line 300 will reduce the value of capacitance C1 as measured fromthe test head 80. FIG. 3 also shows the capacitance C2 between the Awire 302 and earth and the capacitance C3 between the B wire 304 andearth.

Each night, the controller 106 causes the test head 80 to measure theresistances R1, R2, R3, R4, R5, R6 and the capacitances C1, C2, C3 foreach terminating line of the access network 12. The controller 106 alsocauses the test head 80 to check if there is terminal equipmentconnected to the end of the line. Terminal equipment has a standardcapacitance value. When terminal equipment is connected, the value ofits capacitance is subtracted from the capacitance as measured by thetest head to obtain the capacitance C1. For each terminating line, theresults of the tests are stored against its directory number in theaccess network management system 102.

The controller 106 transmits the results of the tests to the accessnetwork management system 102. The access network management system 102examines the results of the series of tests for each terminating linefor the presence of a suspected fault. The possible faults includedisconnection, short circuit, a fault battery voltage, an earth faultand low insulation resistance. When a fault is suspected, the name ofthe fault and the results of the test for the line are stored in theaccess network management system 102 against its directory number or anidentifier in the exchange associated with the line. The details of thesuspected faults found each night may be reviewed by an operator of theaccess network management system 102. Where appropriate, the operatormay give instructions for a fault to be repaired.

The network management system 102 is also arranged to carry out somefurther processing of the data collected from the over-night testing.This further processing is designed to test potential faults rather thanactual faults so that, where appropriate, remedial work can be carriedout before the fault is detected by a customer. An overview of theprocessing carried out by the network management system 102 will now begiven with respect to FIG. 4 and a detailed example of the processingwill also be given below. The processing is initiated at step 401 eitherautomatically in response to the receipt of the appropriate data or by ahuman operator and processing moves to step 403. At step 403, usingknown methods (which will be described in detail below), the test datafor all the lines in question is analysed to identify lines withcharacteristics that indicate that a fault is likely to occur within apredetermined period of time i.e. an anticipated hard fault (AHF). Theparameters for determining this are line resistance measurements and thethresholds are derived from historical data.

At step 405 records of line configurations i.e. the nodes in the networkthrough which particular lines are connected are used to establish thepattern of anticipated hard faults for each node. The pattern is thenanalysed to identify and count clusters of faults in step 407. Then, atstep 409, the clusters for a given node are analysed to verify that thecorrect number of clusters have been identified and that the clustersare statistically significant. At step 411, the clusters of anticipatedhard faults in a given node are used to calculate a cluster node score.This score can then be used to rank the node against other nodes throughwhich the same set of lines pass so as to enable the identification ofthe most likely node from which the faults are emanating. In otherwords, the cluster score can be used to locate the cause of theanticipated faults.

At step 413, further analysis of the anticipated hard faults is carriedout and a priority score calculated for a given node. This priorityscore provides an indication of how soon a node is expected to becomefaulty and is used the establish which one of a set of nodes that carrythe same set of lines is in most urgent need of attention. It should benoted that the cluster score and the priority score can be usedindependently or in combination. In other words, in carrying outpreventative maintenance on a given node, the indication of the nodemost likely to be the source of the anticipated hard faults can be usedindependently or in combination with the indication of the node which islikely to become most faulty soonest.

The invention will now be described further by way of a worked exampleshowing test data from a set of lines being processed in the manneroutlined above with reference to FIG. 4. FIG. 5 shows the test data foreach of twenty six lines running from an exchange. For each line thetest data comprises four capacitance measurements between the A wire andearth, between the A wire and the B wire (both a current measurement anda prior measurement) and between the B wire and earth. The data alsocomprises a distance measurement for each line and a series ofresistance measurements between each combination of the A wire, B wire,Battery and Earth. These correspond to the capacitances C1, C2, C3 andresistances R1, R2, R3, R4, R5, R6 described above with reference toFIG. 3. In addition, there is a previous capacitance reading between theA and B wires and a termination flag (Term) supplied by the Vanderhoffand/or Terradyne equipment. However, for the purposes of the presentinvention, only the resistance measurements between the A wire, the Bwire and the Battery i.e. R4 and R6, are used.

From historical data a threshold limit is defined for the measurementsR4 and R6 below which the line to which the measurements apply istreated as having an anticipated hard fault (AHF). An anticipated hardfault is defined as a line which is expected, on the basis of its R4 andR6 resistances, to become faulty (i.e. a hard fault) with apredetermined period. In the present embodiment the predefined period isone year and the limit for the resistance measurements is 400 kohms.This threshold may be determined by analysis of historical data forlines which have become faulty. Alternatively the threshold can beestimated and then adjusted while the system is in use.

A noted above, FIG. 5 shows the test data for the lines emanating froman exchange. It can be seen that lines 4, 5, 9 to 12, 16 to 18, 20, 21,23 and 24 all have resistance measurements between the A or B wire andbattery of less than 400 kohms and as a result are classified asanticipated hard faults. FIG. 6 is a schematic representation showingnine of the twenty six lines 601 to 609 of FIG. 5 as they emanate froman exchange 610 to the exchange side of a primary connection point (PCP)cabinet 611, to the distribution side 612 of the PCP, to twodistribution points (DPs) 613, 614 and on toward customer premisesequipment (CPE) (not shown). Only nine of the twenty six lines are shownin FIG. 6 for the sake of clarity.

Each of the connection points on the exchange 610, the PCP 611, 612 andthe DPs 613,614 is individually identified by a letter and numbersequence as shown in FIG. 6. These connection identifiers enable theroute that each line takes through the network nodes to the CPE to berecorded. Accordingly, each line 601-609 has a data record associatedwith it that is stored in the access network management system 102. Therecord for each line shows data such as the telephone number associatedwith the line and the connection identifiers for each line. For example,the connection identifiers for line two 602 in FIG. 6 would be A03, E07,D08 and DP10. These identifiers are also associated with a uniqueidentification of the node in the network to which they apply so as toenable connection identifiers on two nodes of the same type to be toldapart such as those on the two DPs 613, 614 shown in FIG. 6.

As will be understood by those skilled in the art, lines from anexchange to the CPE seldom follow an orderly path through the nodes ofthe network. In other words a line will not be connected to point A01 inthe PCP, then E01, D01 and DP01 but instead will take an effectivelyrandom route across the connection points. In some cases, lines aredeliberately mixed up so as to reduce the problems of cross-talk betweenthe cables i.e. in an attempt to avoid two or more cables running alongthe same physical path. This mixing up is carried out for example in theconnections between the E-side and the D-side of a PCP such as PCP 611,612 in FIG. 6.

An anticipated hard fault (AHF) that is identified on a particular linemay have occurred as a result of degradation of the line at any pointalong its length from the exchange to the CPE. Faults (including AHFs)very often occur at the points where the line is connected to a networknode such as a PCP or DP. These are points at which the physical cableis more easily affected by corrosion, the breakdown of insulation orwater ingress. In FIG. 6, the points at which the lines that are showingan AHF according to the test data of FIG. 5 are connected to networknode are indicated with large black dots (∘). As noted above, not allthe lines emanating from the exchange 610 are shown but instead and nineexample lines are shown.

As noted above, the first step 403 in the processing carried out by thenetwork management system 102 is to identify the lines that show AHFsand this is carried out by the analysis of the data shown in FIG. 5.This analysis reveals AHFs on lines 2 and 5 to 8 in the present example.In the next step 405, the processing analyses each node or each one of aselection of nodes from the network. This analysis will now be explainedfurther with reference to an example of 28 cables from a frame of anetwork node (the node could be an exchange, a PCP or a DP). The frameis represented in table 1 below by a sequence of nominal connectionidentifiers 1 to 26:

TABLE 1

The second line of table 1 above determines whether or not the lineattached at the relevant connection point is exhibiting an AHF. An “a”designates a fault free line while a “b” designates a line exhibiting anAHF. The next step in the processing to establish the number of clustersof AHFs that are present for the frame. Firstly the range over which AHFclusters occur is established. In the example of table 1 above theclusters start at line 4 and extend to line 24. Therefore the clusterrange is 4 to 24 and of these lines 13 are showing AHFs (i.e. aresuspect).

The next step 405 in the processing determines whether any of the lineswhich are not shown as AHF that are between groups of suspect lines are,in fact, misdiagnosed and should be treated as “b”s or AHFs. The basisfor this element for the processing is that lines or cables that aresituated in close proximity tend to share fault characteristics sincethe cause of the fault in one line, for example water dripping down theframe of the network node, is not in practice isolated to that singleline or cable. The Cluster Range i.e. the number or distance between twosuspects (“b”s) that determines whether or not the two suspect are partof the same cluster or are separate clusters is determined in accordancewith the following formula:Cluster Range=(No. in Group/No. of Suspect)^(P)(where “p” is the range parameter which in the present embodiment is setto 0.5)

The formula refers to a group which is a subset of the data from table 1selected from the first line exhibiting an AHF to the last line to doso. In table 1 above, the group will run from position four to position24. The formula takes the total number of suspect in the group beinganalysed, divides it by the total number of suspects in the group andmultiplies this to the power of the range parameter p. Therefore in thepresent example, the cluster range is calculated as (24/13)^(0.5)=1.84.The cluster range is then used to determine which of the apparentlyfault free lines (“a”s in table 1 above) that are physically locatedbetween lines that show AHFs should be treated as showing an AHF. Inother words, if there is only one “a” between two (or more) “b”s thenthe “a” is treated as a “b” and part of the cluster with its adjacent“b”s i.e. 1<cluster range=1.84. If there were two “a”s then these wouldnot be treated as forming a cluster with the adjacent “b”s i.e.2>cluster range=1.84. Applying the cluster range to the results shown intable 1 has the following results illustrated in table 2 below.

TABLE 2 Position on Cluster ID Cluster ID Below Cluster Cluster TypeFrame Number (B) Number (A) Range? B 4-5 1 A 6-8 2 N B  9-12 3 A 13-15 4N B 16-18 5 A 19 5 Y B 20-21 5 A 22 5 Y B 23-24 5 Total B  3 ClustersTotal A  2 Clusters

The result of the application of the cluster range to the data fromtable 1 can be seen in the fifth column of table 2. This shows that the“a”s at positions 19 and 22 of table 1 have been treated as “b”sresulting in the data from positions 16 to 24 being treated as a singlecluster of AHFs. Conversely, the “a”s at positions 6 to 8 and 13 to 15are treated as legitimately indicated as fault free i.e. not part oftheir adjacent fault clusters.

Accordingly, the information recovered from the analysis of the data oftable 1 is as set out below in table 3.

TABLE 3 Number of Suspects (AHF) 13 NS Number of Clusters (A & B)  5 NCNumber in Group (24-4) + 1 21 Number of None Suspect 21-13  8 NO

The total number of lines identified as suspect is thirteen and make upa total of five clusters. The total number of lines in the group is 21i.e. excluding from the data in table 1 the non-faulty lines at thebeginning and end of the sequence. The total of non-suspect lines withinthe group is eight. In determining the data in table 3 above, the linesat positions 19 and 22 are treated as “b”s for the cluster scorecalculation but as “a”s for the remaining calculations.

The next stage 409 in the processing is to determine whether theclustering that has been identified is coincidental or more likely toresult from a single cause. Essentially the test is one of randomness.If the cluster pattern is random then it is treated as coincidentalwhile if it is not random it is treated as resulting from a singlecause. This is determined by calculating a cluster value as follow:

${{Cluster}\mspace{14mu}{Value}} = {{ABS}( \frac{{NC} - {Mean}}{SD} )}$

Where NC is found in table 3 above, SD is the standard deviation set outbelow along with the formula for the Mean.

${Mean} = {( \frac{2 \times {NS} \times {NO}}{{NS} + {NO}} ) + 1}$${SD} = \sqrt{\frac{2 \times {NS} \times {{NO}( {{2 \times {NS} \times {NO}} - {NO} - {NS}} )}}{( {{NO} + {NS}} )^{2} \times ( {{NO} + {NS} - 1} )}}$

These equations make up a test called the Mann Whitney U Test which is atest for randomness. Taking the data recovered and shown in table 3above, the following calculations are made by the processing in step409:

${Mean} = {{( \frac{2 \times 13 \times 8}{13 + 8} ) + 1} = 10.904}$${SD} = {\sqrt{\frac{2 \times 13 \times 8( {{2 \times 13 \times 8} - 8 - 13} )}{( {8 + 13} )^{2} \times ( {8 + 13 - 1} )}} = 2.099}$${{Cluster}\mspace{14mu}{Value}} = {{{ABS}( \frac{5 - 10.904}{2.099} )} = 2.853}$

The cluster value is then compared to a threshold value called thecluster parameter. If the cluster value is above the threshold thecluster in question is treated as a valid cluster. If the cluster valueis below the threshold then it is not treated as a cluster. In thepresent embodiment, the cluster parameter is set at 1.96 which is thepoint at which there is a 95% chance of the pattern of AHFs beingnon-random according to a normal distribution. The cluster parameter canbe adjusted while the system is in use. It can be seen that in thepresent example, the cluster value of 2.853 is greater that the clusterparameter thus indicating that the data from table 1 being analysedrepresents a true (i.e. non-random) cluster.

The next step 411 in the processing of the is to calculate the priorityscore for the node being analysed. This score takes in to account anumber of different factors of historical data relating to the nodebeing analysed as well as the cluster value established in the previoussteps to calculate a priority score for the node. The data used by thisstep in the analysis is, in the present embodiment, stored by thenetwork management system 102 for each node and comprises the number oflines that are not being used i.e. the number of spare pairs, the numberof suspect lines (or pairs), the number of working lines, the number offaulty lines, a previously retained percentage increase in faulty lines.The following formulae is then used to calculate the priority score forthe node.

${PriorityScore} = {( {1 - {( \frac{S - ( {{Sus} + F} )}{{One}(S)} ) \times ( \frac{{Sus} \times 100}{{One}(W)} )}} ) + ( {I \times {P1}} ) + ( {C \times {P2}} )}$

-   -   where:    -   S=spare lines;    -   Sus=suspect lines;    -   W=working lines;    -   F=faulty lines;    -   C=cluster value;    -   I=percentage increase in faulty lines; and    -   I^(P)=previous percentage increase in faulty lines.

The percentage increase in faulty lines I is calculated in accordancewith the following formulae:

$I = {( \frac{F}{F + S + W} ) - I^{P}}$

-   -   if I<1 then

There are two further factors P1 and P2 which affect the priority score.These are weighting factors which can be used to adjust the performanceof the priority algorithm. The first weighting factor P1 is termed theFault Increase Weighting Factor and in this embodiment is set to a valueof 100. I is a measure of the rate of fault increase and P1 governs theeffect that I has on the priority score. The second weighting factor P2is termed the Grouping Algorithm Weighting Factor and in this embodimentis set to a value of 10. P2 governs the effect that the cluster value Chas on the priority score. The priority score algorithm also makes useof a function called “One” which converts values of “0” to “1”.

The calculation of the priority score will now be explained further withreference to an example of the E-side of a PCP that has 87 lines (orpairs) running in to in, 10 are spare lines, 13 are suspect (AHFs), 65are working (i.e. not faulty or AHFs) and 12 are known to be faulty. Thesuspects in this example of 87 lines are clustered in the same patternas show in table 1 above. The cluster value calculation is independentof the number of lines and instead only takes in to account the lines inthe suspect group. As a result, the cluster value for the presentexample of 87 lines will be the same as that calculated above withreference to the data of table 1 i.e. 2.853. In this example theprevious percentage increase in faulty pairs is 12.6%.

Accordingly, in step 411, I is calculated as follows:

$I = {{( \frac{12}{12 + 10 + 65} ) - 0.126} = {{0.137 - 0.126} = 0.011}}$

Thus the priority score for the node is calculated as follows:

$\begin{matrix}{{PriorityScore} = {( {1 - {( \frac{10 - ( {13 + 12} )}{{One}(10)} ) \times ( \frac{13 \times 100}{{One}(65)} )}} ) + ( {0.011 \times 100} ) +}} \\{= {( {1 - ( {{- 1950}/650} )} ) + 1.10 + 28.53}} \\{= {4.00 + 1.10 + 28.53}} \\{= 33.63}\end{matrix}( {2.853 \times 10} )$

As noted above, the priority score is calculated for a number of nodesin the network and can then be used to determine how work such aspreventative maintenance should be prioritised. The higher the priorityscore, then the more urgent the maintenance. FIG. 7 shows the same setof network nodes as are described above with reference to FIG. 6 butwith the addition of the priority scores and cluster scores for eachnode. Again, only nine of the 87 lines running from the exchange areillustrated for the sake of clarity.

The node with the highest cluster score and the highest priority scoreis the E-side of the PCP 611. This indicates to the network managerthat, because there is a cluster of faults in that node, it is likely tobe the source of the anticipated faults that have been detected on thelines that run through the set of nodes that have been analysed. Often,as mentioned above, such clustered AHFs are caused by the same problemsuch as water leaking in to the cabinet that holds the network node andcausing corrosion and/or short circuits. The priority score gives thenetwork manager a further indication of how the maintenance of thenetwork of FIG. 7 should be planned as it gives a relative measure ofthe urgency of the preventative maintenance for a given node. In otherwords it gives an indication of how soon hard faults are going to appearand how many.

In the example shown in FIG. 7, the highest priority score and thehighest cluster score both occur for the same node. Although this willnot be an unusual situation in practice, situations are also possiblewhere the highest of each of the scores occur for different nodes. Inthis case, the judgement of the network manager would be needed todecide between carrying out maintenance on the highest priority node orthe node with the greatest cluster score (or perhaps both). It will alsobe clear to those skilled in the art that the cluster score system andthe priority score system can be used either together as noted above orindependently of each other. Furthermore, although the cluster value isused in the calculation of the priority score for a node, it will beclear to those skilled in the art that this is not essential and that apriority score, for use in the same manner as described above can stillbe calculated without taking in to account a cluster value.

The results of the processing of the data of table 1 to produce thecluster and priority scores for each node in the network can bepresented to the user of the network management system 102 in a numberof ways. For example, the results can be presented in tabular form withcolumns showing the scores for each node. Alternatively, the results canbe displayed pictorially as shown in FIG. 7 with the scores beingpresented in boxes near a representation of the network node to whichthey relate. This can be supplemented by indications such as the blackdots (∘) where lines exhibiting AHFs are attached to the network nodesso as to give a visual indication of the clustering in addition to thecluster score.

Although the present invention has been described with reference to anaccess network in which each circuit is carried by a piece of copperwire, it may also be used for terminating circuits carried by opticalfibres.

It will be understood by those skilled in the art that the apparatusthat embodies the invention could be a general purpose computer havingsoftware arranged to provide the analysis and/or processing of the testdata. The computer could be a single computer or a group of computersand the software could be a single program or a set of programs.Furthermore, any or all of the software used to implement the inventioncan be contained on various transmission and/or storage mediums such asa floppy disc, CD-ROM, or magnetic tape so that the program can beloaded onto one or more general purpose computers or could be downloadedover a computer network using a suitable transmission medium.

Unless the context clearly requires otherwise, throughout thedescription and the claims, the words “comprise”, “comprising” and thelike are to be construed in an inclusive as opposed to an exclusive orexhaustive sense; that is to say, in the sense of “including, but notlimited to”.

1. A method of operating a fault management system for a communicationsnetwork comprising a plurality of lines passing through a plurality ofnodes, said method comprising: performing a test on a plurality of saidlines to obtain one or more elements of test data for each line;analyzing the test data to identify lines with common faultcharacteristics; establishing a score for each node based on a count oflines with common fault characteristics, which score provides a relativemeasure of urgency of maintenance tasks required to remove effects ofline fault characteristics from the node; and establishing a score foreach node based on a relative measure of physical clustering of lineswith common fault characteristics for each node so as to give anindication of the node at which the cause of a common faultcharacteristic is most likely to be located.
 2. A method as in claim 1in which, the scores for each node are calculated using numbers of:spare lines, working lines, suspected faulty lines and actual faultylines.
 3. A method as in claim 1 in which the score based on clusteringis used in calculation of a priority score.
 4. A method as in claim 3 inwhich calculation of the priority score depends on a percentage increasein faulty lines for the node.
 5. A tangible computer program storagemedium containing a computer program or set of computer programsarranged to cause a general purpose computer or group of such computersto carry out the method of claim
 1. 6. A fault management system for acommunications network, the network comprising a plurality of linespassing through a plurality of nodes, said system comprising: meansoperable to perform a test on a plurality of said lines to obtain one ormore elements of test data for each line; means operable to analyze thetest data to identify lines with common fault characteristics; meansoperable to establish a score for each node based on a count of lineswith common fault characteristics, which score provides a relativemeasure of urgency of maintenance tasks required to remove effects ofline fault characteristics from the node; and means for establishing ascore for each node based on a relative measure of physical clusteringof lines with common fault characteristics for each node so as to givean indication of the node at which the cause of a common faultcharacteristic is most likely to be located.
 7. Apparatus as in claim 6in which, the scores for each node are calculated using numbers of:spare lines, working lines, suspected faulty lines and actual faultylines.
 8. Apparatus as in claim 6 further comprising means for using thescore based on clustering in calculation of a priority score. 9.Apparatus as in claim 8 in which calculation of the priority scoredepends on a percentage increase in faulty lines for the node.
 10. Atangible computer program storage medium containing a computer programor set of computer programs arranged to cause a general purpose computeror group of such computers to provide the system of claim 6.